I do not have a doctorate but for you Ernie, I'll try. I am reading this from the article "The Eyes Have It". It is dedicated to coyotes but goes on to explain that this is the same with all canines.
Both humans and canines have the same rod photopigment. During the day, the cones regulate sight, as rods are fully saturated with light. At night the rods take over. In order to spot their quarry in the dark, coyotes need more rods than cones.
To accomodate the need for more rods, coyote eye biology has adapted and something had to go - one of the cones. The superfluous cone was the green cone which partially overlaps with the red cone in terms of special sensitivity. As noted the extra rods give the coyote far greater night vision than humans. Add the larger pupil diameter and the larger lens, and coyotes see roughly six times beter than humans at night. But also noted, rods permit poor visual acuity which gives them poor spacial resolution. This means that coyotes have slightly blurred vision and are essentially, nearsighted. This does not mean coyotes have poor eyesight, just the opposite. Canines are designed perfectly for what they do best, pinpointing objects in low light and spotting the slightest movement.
Hope this helps to explain my ramblings a little better as I am always learning myself........Tim
